1. Field of the Invention
The present invention relates to a photoelectric conversion device and an image pickup device that perform photoelectric conversion and use a field emission type electron source.
2. Description of the Related Art
Application of a strong electric field to a solid surface lowers a potential barrier and reduces the width of the potential barrier of the solid surface confining the electrons within the solid, so that electrons (cold electrons) are emitted by a tunneling effect. This phenomenon is called the field emission. Field emission devices (FEDs) known are Spindt-type devices, surface conduction electron emitters (SCEs), devices having a metal-insulator-metal (MIM) structure or metal-insulator-semiconductor (MIS) structure, and so forth. Of these, Spindt-type devices each having a conical cold cathode are the most widely known.
Development of image pickup devices using the field emission devices has been underway in recent years. For instance, Japanese Patent Kokai No. 2000-48743 discloses an image pickup device using Spindt-type devices. FIG. 1 is a cross section schematically illustrating a structure of an image pickup device 100 disclosed in Japanese Patent Kokai No. 2000-48743. Referring to FIG. 1, the image pickup device 100 has a vacuum vessel 102 consisting of a cathode substrate (glass substrate) 101, a transparent substrate 103 and spacers 104A and 104B. Field emission devices 111 and a photoelectric conversion target 120 are disposed within this vacuum vessel 102. The photoelectric conversion target 120 includes a conductor film 121 and a photoelectric conversion film 122 which is formed on the back side of the transparent substrate 103. The field emission devices 111 each have a cathode conductor 113, a conical emitter (cold cathode) formed on this cathode conductor 113, and a gate electrode 112. The distal ends of the emitters 114 are exposed through openings formed in the gate electrodes 112, and face the photoelectric conversion target 120. A mesh electrode (sealed grid electrode) 110 is disposed between the photoelectric conversion film 122 and the field emission devices 111. When a high voltage is applied to the gate electrodes 112, electron beams are emitted from the distal ends of the emitters 114 by the above-mentioned field emission, and these beams pass through holes in the mesh electrode 110 and reach the photoelectric conversion film 122.
The photoelectric conversion film 122 is a film containing a sensitizer, and is under the application of a strong electric field. Incident light on the photoelectric conversion film 122 generates electron-hole pairs within the film, resulting in an avalanche phenomenon that amplifies the holes. When these holes recombine with the electrons arriving from the field emission devices 111, current flows so as to replenish the electrons annihilated via the recombination, so the amount of incident light on the photoelectric conversion film 122 can be measured by detecting this current.
The emitters 114 of the field emission devices 111 are arranged in a matrix, and the field emission devices 111 are driven in a dot sequential manner for every pixel. The reason for driving the field emission devices 111 in a dot sequential manner is that since the photoelectric conversion film 122 is a continuous film, we do not know which pixel the detected signal corresponds to when two electron beams are emitted simultaneously from emitters 114 whose pixel positions are apart from each other. In general, the higher is the resolution of the image pickup device 100, the more pixels there are, so when the field emission devices 111 are driven dot-sequentially, a drive time per pixel ends up being extremely short. For example, in the case of VGA specification, a resolution of the image pickup device 100 is 640×480 pixels. To scan all of the pixels in one frame within 1/30 of a second, the drive time per pixel has to be an extremely short time of only about 100 nanoseconds.
However, with the above-mentioned image pickup device 100, because drive circuits 106A and 106B that drive the field emission devices 111 are disposed on a cathode substrate 101 in parallel with the field emission devices 111, there are many signal lines connecting the drive circuits 106A and 106B and the field emission devices 111. This tends to generate cross-talk between the signal lines and delay the drive signals transmitted along the signal lines. Since the drive time per pixel is extremely short, as mentioned above, a delay in the drive signals is a problem in that dot-sequential scanning cannot be accurately executed.
Furthermore, high-frequency drive signals outputted from the drive circuits 106A and 106B also impart considerable cross-talk to the photoelectric conversion film 122, and this lowers S/N ratio of the signals detected from the photoelectric conversion film 122, resulting in the problem of inferior quality of the image signal.